How does a quadrupole mass spectrometer work?
The ion beam extracted from the electron impact ion source is diverted into a quadrupole separation system containing four rod-shaped electrodes. The cross sections of the four rods form the circle of curvature for a hyperbola so that the surrounding electrical field is nearly hyperbolic. Each of the two opposing rods exhibits equal potential, this being a DC voltage and a superimposed high-frequency AC voltage (Fig. 4.2). The voltages applied induce transverse oscillations in the ions traversing the center, between the rods. The amplitudes of almost all oscillations escalate so that ultimately the ions will make contact with the rods; only in the case of ions with a certain ratio of mass to charge m/e is the resonance condition which allows passage through the system satisfied. Once they have escaped from the separation system the ions move to the ion trap (detector, a Faraday cup) which may also take the form of a secondary electron multiplier pick-up (SEMP).
- Focusing plate (extractor diaphragm)
- Ion source exit diaphragm (total pressure measurement)
- Quadrupole exit diaphragm
The length of the sensor and the separation system is about 15 cm. To ensure that the ions can travel unhindered from the ion source to the ion trap, the mean free path length inside the sensor must be considerably greater than 15 cm. For air and nitrogen, the value is about p · λ = 6 · 10–3 mbar · cm. At p = 1 · 10-4 bar this corresponds to a mean free path length of λ = 60 cm. This pressure is generally taken to be the minimum vacuum for mass spectrometers. The emergency shut-down feature for the cathode (responding to excessive pressure) is almost always set for about 5 · 10-4 mbar. The desire to be able to use quadrupole spectrometers at higher pressures too, without special pressure convertors, led to the development of the XPR sensor (XPR standing for extended pressure range). To enable direct measurement in the range of about 2 · 10-2 mbar, so important for sputter processes, the rod system was reduced from 12 cm to a length of 2 cm. To ensure that the ions can execute the number of transverse oscillations required for sharp mass separation, this number being about 100, the frequency of the current in the XPR sensor had to be raised from about 2 MHz to approximately 6 times that value, namely to 13 MHz. In spite of the reduction in the length of the rod system, ion yield is still reduced due to dispersion processes at such high pressures.
Additional electronic correction is required to achieve perfect depiction of the spectrum. The dimensions of the XPR sensor are so small that it can “hide” entirely inside the tubulation of the connection flange (DN 40, CF) and thus occupies no space in the vacuum chamber proper. Fig. 4.1a shows the size comparison for the normal high-performance sensors with and without the Channeltron SEMP, the normal sensor with channelplate SEMP. Fig. 4.1b shows the XPR sensor. The high vacuum required for the sensor is often generated with a TURBOVAC 50 turbomolecular pump and a D 1.6 B rotary vane pump. With its great compression capacity, a further advantage of the turbomolecular pump when handling high molar mass gases is that the sensor and its cathode are ideally protected from contamination from the direction of the forepump.
a: High-performance sensor with Channeltron
b: Compact sensor with Micro-Channelplate
c: High-performance sensor with Faraday cup
Design of the sensor
One can think of the sensor as having been derived from an extractor measurement system (see Fig. 4.3), whereby the separation system was inserted between the ion source and the ion trap.
- Ion trap
The normal (open) ion source
The ion source comprises an arrangement of the cathode, anode and several baffles. The electron emission, kept constant, causes partial ionization of the residual gas, into which the ion source is “immersed” as completely as possible. The vacuum in the vicinity of the sensor will naturally be influenced by baking the walls or the cathode. The ions will be extracted through the baffles along the direction of the separation system. One of the baffles is connected to a separate amplifier and – entirely independent of ion separation – provides continuous total pressure measurement (see Fig. 4.4). The cathodes are made of iridium wire and have a thorium oxide coating to reduce the work associated with electron discharge. (For some time now the thorium oxide has gradually been replaced by yttrium oxide.) These coatings reduce the electron discharge work function so that the desired emission flow will be achieved even at lower cathode temperatures. Available for special applications are tungsten cathodes (insensitive to hydrocarbons but sensitive to oxygen) or rhenium cathodes (insensitive to oxygen and hydrocarbons but evaporate slowly during operation due to the high vapor pressure.)